Mobile machines such as excavators, bulldozers, and graders are commonplace in applications such as construction and mining. These machines include a tool, such as a bucket or a blade, which is positioned by a tool positioning mechanism under the control of a human operator or an on-board tool control system. For example, referring to FIG. 1, which shows an excavator 1, the tool may be a bucket 2, and the tool positioning mechanism 3 may comprise an arm 5 and a boom 6, to which the bucket 2 is mounted.
To allow accurate positioning of the tool, an on-board tool control or guidance system may require precise knowledge of the position of the tool relative to a reference position, such as the ground level. Accordingly, on machines such as those mentioned above, the tool positioning mechanism sometimes is equipped with linear or angular position encoders or other similar sensors. The outputs of the encoders are provided to the tool control system to indicate the physical configuration of the positioning mechanism and the position of the tool. A common problem is that, over time, position inaccuracy tends to accumulate in the linkages of the tool positioning mechanism. This inaccuracy tends to be reflected in the outputs of the encoders. Because the outputs of the encoders can be crucial in positioning the tool, it may be desirable to calibrate the encoders before beginning a task, to ensure that the tool control system uses reliable data. The encoders may be calibrated by setting their outputs to a known value when the position of the tool is precisely known, relative to a reference (e.g., the ground level).
One possible calibration approach involves distributing a number of stakes or other similar reference points around the job site, each of which has a visible mark at a precisely known height. The machine is driven close to a stake, the tool is aligned with the mark, and the tool positioning system is then calibrated. This approach is undesirable, however, because it requires time and manpower to survey the stakes. In addition, the tool positioning system can only be calibrated when the machine is located next to one of the stakes.
Another calibration technique uses a laser beam at a fixed, precisely known height as a reference level. The laser beam is detected by a photosensor on-board the machine. The use of a laser beam, however, is subject to line of sight limitations. Thus, obstacles on the job site can hinder calibration of the tool control system when such a technique is used.
What is desired, therefore, is a technique for calibrating a tool positioning system on a mobile machine which overcomes these and other disadvantages. In particular, what is desired is a technique for calibrating a tool positioning system on a mobile machine in a stakeless environment without the line of sight limitations associated with